As a conventional thermal flowmeter, a flow measurement device, which includes a sub-passage disposed in a main passage through which a fluid flows and taking a part of the fluid; a flow measurement element disposed inside the sub-passage and having a heating resistor pattern formed therein; and a support on which the flow measurement element is mounted, is known (see claim 1 and the like of the PTL 1 to be described below).
The conventional flow measurement device includes a first fluid passage portion and a second fluid passage portion. The first fluid passage portion is constituted by a surface on which a flow measurement element is mounted, and a passage forming surface of the sub-passage. The second fluid passage portion is constituted by a surface on the opposite side of the surface on which a flow measurement element is mounted, and the passage forming surface of the sub-passage.
In the conventional flow measurement device, the passage forming surface of the first fluid passage portion opposing the flow measurement element on the upstream side of the flow of the fluid has an inclined surface that causes the flow of the fluid to be oriented the flow measurement element. This inclined surface is constituted by two or more surfaces with different orientations.
With the above configuration, it is possible to suppress dust from being bounced back by the inclined surface provided on the opposing surface on the upstream side of the pattern of the heating resistor of the fluid passage portion on the heating resistor pattern side and then flowing toward the pattern of the heat generating resistor together with the flow of the fluid. Thus, it is possible to suppress a breakage or contamination of the flow measurement element configured using the heating resistor pattern, and to provide a highly reliable flow measurement device which is excellent in dust resistance even in an unsteady flow field such as pulsating flow and in which a characteristic error hardly occurs (see paragraph 0009 and the like of the same literature).